First off the Steelers don't have the best OL, but they cut G/T Chris Scott and the Packers signed him to their practice squad just this year... and signed Kuhn in years past. So it wouldn't be totally surprise to see the Packers pick-up a former Steeler.
I think Ta'amu has the talent, he's just gotten a bit lazy lately (which if you follow NTs, isn't uncommon), remember NT Howard Green who helped the Packers win the Super Bowl, that's basically his whole career... getting in shape long enough to make the team, then falling out of shape.
Ta'amu early years in college weight got WAY up there and got way out of shape. Coaches got him and he got into a lot better shape and according to some looked like a 1st round NT (because he was a pass rushing 350 pound guy which often get over rated at the college level). Then he got senioritis his senior year of college and his stock and play when down... and rumored to report to the Steelers out of shape and over weight... then got into trouble...
My point being is there is a pattern with NTs... and Ta'amu fits in it... gets out of shape their first years until coaches get on them, then they get into shape, then when they think they're safe get out of shape if the coaches don't stay on them.
Ta'amu is very out of shape right now, and wants to act like a pass rushing DT in the pros (which he's not and his body size and play style won't let him be on a full time level) but you get coaches to get on him and stay on him, and I think he'll get into shape and start trying to level how to act and play like a run stuffer instead of the pass rushing DT which he was able to get away with in college (and his coaches might of asked him to do) but won't get away with in the pros.
At the very least I think a practice spot with close to full roster pay is worth it just to see if he has the potential going forward to keep around and see if he can stay out of trouble. Because if he can he could be a long term back-up to Raji or better... if he gets back into his really good shape he might be able to rotate other places as well and maybe help with the nickel package.
When he was in good shape his bull rush was much better than Pickett moves. Of course Pickett is much better run stuffer because Ta'amu doesn't know how to (yet).